A frittata is an Italian egg cake similar to Spanish tortilla. Start this on a burner in a skillet with a heatproof handle, than transfer it to the oven for a nice puffy texture.
3 tablespoons olive oil
1 small red onion, thinly sliced
1 red bell pepper, thinly sliced
1 teaspoon kosher salt
6 ounces baby spinach
1/2 cup whole milk
3/4 cup freshly grated Parmesan
8 fresh basil leaves, coarsely chopped
1/2 teaspoon black pepper
Set the oven at 450 degrees.
In a large nonstick skillet with a heatproof handle, heat 1 1/2 tablespoons of the olive oil over medium heat. Add the onion, bell pepper, and 1/2 teaspoon of the salt. Cook, stirring often, for 10 minutes or until the vegetables soften.
Add the spinach and cook, stirring, for 3 minutes or until it just wilts. Remove from the heat and cool for a couple of minutes.
In a large bowl, beat the eggs, milk, Parmesan, basil, black pepper, and the remaining 1/2 teaspoon salt. Stir in the vegetables.
Wipe out the skillet with a paper towel. Heat the remaining 1 1/2 tablespoons oil over medium heat. Add the egg mixture and cook for a couple of minutes, shaking the pan occasionally, until the eggs start to get firm around the sides.
Transfer the skillet to the oven and cook for 12 minutes or until the frittata is puffy and brown and cooked in the center when pierced with a paring knife.
Serve hot, warm, or at room temperature, cut into wedges or squares.